A new ultrasonic instrumentation system was developed and applied to the problem of detecting critical heat flux (CHF) in experiments that simulate a nuclear reactor fuel assembly. This instrumentation system used the principles of ultrasonic thermometry to detect and locate CHF in a tube bundle with non-uniform axial heat generation. The technique consists of measuring the time between pairs of ultrasonic echoes that reflect from a sequence of evenly spaced discontinuities along a sensor. Each measurement of time is directly related to the temperature of a specific segment of a sensor. The system was designed to handle many 16-zone sensors at a high rate of data acquisition so that CHF could be rapidly detected and accurately located. This paper includes a description of the sensor and the signal processing techniques as well as examples of the system’s response to CHF.

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